API endpoint for fetching a single codelist

Add an API endpoint to fetch a single codelist directly by slug.

There is no current client for this in the OpenSAFELY systems, but it seems plausibly useful to those interested in using the OpenSAFELY API in scripting and tooling. Part of the "Open" ethos is that the resources that are publicly human-accessible on the site should be conveniently accessible in machine-readable form via an API.

It would be useful in https://github.com/ebmdatalab/codeusage/, allowing it to avoid needing to decode the organization from the slug and fetch all codelists in the organization then find the specific codelist within the response. This is slowing down this tooling and slightly inefficient for our resource consumption.

Start by locating the existing API routes and response handling for codelists, then inspect how codelist slugs and organization lookups are represented. Define the single-codelist endpoint and confirm its response matches the machine-readable API conventions. Done means a codelist can be fetched directly by slug without fetching all codelists in an organization, with coverage for the endpoint behavior.